Such a tap changer is already known from the company publication of the applicant “Umsteller DEETAP® U, Betriebs-anleitung BA 249/02de,” printing 0204/200. This known tap changer operates according to the principle of the rotary tap changer. It comprises a dielectric housing on which fixed contacts are arrayed in circles in each of several horizontal planes. A rotatable insulated switching shaft is disposed in the interior of this dielectric housing and carries in each horizontal plane at least one switching contact that can make an electrical connection with the corresponding fixed contacts in different ways depending on the respective basic circuit, for example as a star-point tap changer, simple center tap changer, double center tap changer, series-parallel tap changer or star-delta tap changer.
The adjusting of the tap changer from one operating setting to another, i.e. the switching between different winding taps of the connected tapped transformer, is carried out by rotation of the insulated switching shaft when the transformer is switched off, i.e. in the current-free and voltage-free state. Rotation of the insulated switching shaft is usually effected by a stepping drive having an electric motor drive or a mechanical manual drive acts as shall be explained in more detail in the following.
The upper closure of the known tap changer forms the tap changer head with the attached upper transmission stage and the Geneva transmission, which is disposed thereunder, for driving the insulated switching shaft. A shaft extending from the motor or manual drive is connected with an outer shaft coupling at the upper transmission stage; the rotational movement of the shaft is transmitted to the upper transmission stage by way of a worm gear on a drive shaft which is vertically mounted in the tap changer head and which in turn has a Geneva crank at its lower end. The Geneva crank engages in a Geneva wheel fixed on the insulated switching shaft at the upper end.
A switching step, i.e. a rotation of the insulated switching shaft through a defined angle with respect to the respective adjacent fixed contact, thus corresponds with a defined number, which is dependent on type, of revolutions of the driving shaft of the motor or manual drive.
Certain variants of the known tap changer have instead, as a special construction, a direct mechanical actuation possibility of the insulated switching shaft by a handwheel, which is connected therewith and arranged centrally on the tap changer head, or a hex fitting in conjunction with a socket wrench. Due to the direct central connection a special tap changer head is required for these constructions. This leads to an undesired variant multiplication of necessary pads of an otherwise identical tap changer.